Cell searcher and cell searching method

ABSTRACT

Peak detector  104  detects the first peak timing where the correlation value of the first synchronization channel is the greatest. SSCH identifier  108  identifies the second synchronization channels by adding the correlation values of the second synchronization channels at a number of timings, and detects the timing where the sum of the correlation values of the identified second synchronization channels is the largest as a second peak timing. Based on the first and second peak timings, timing controller  105  instructs SSCH identifier  108  on the timing to perform correlation calculation. Likewise, based on the first and second peak timings, frequency corrector  110  calculates the error in a frame time measured by a base station, and converts this into a frequency difference, and changes the frequency of an oscillator in RF receiver  102  in such a way that corrects the frequency difference. By this means second synchronization channels and mid-ambles are accurately identified even when a base station gives a different measurement of a frame time.

TECHNICAL FIELD

[0001] The present invention relates to apparatus and methods for cell search, used in mobile communication systems of W-CDMA/TDD schemes.

BACKGROUND ART

[0002] In a mobile communication system, a communication terminal apparatus, when the power is turned on, searches for a cell to which it belongs (i.e. initial cell search), and searches for the cells as it moves across the cells (i.e. idle period cell search). Below, a cell search method in a mobile communication system of a W-CDMA/TDD scheme will be explained.

[0003] Every cell in a mobile communication system is assigned a scrambling code and a code group that corresponds to the scrambling code. As for the code group, there are four combinations of mid-ambles and scramble codes assigned such that they are not erroneously identified between neighboring cells.

[0004] Moreover, as shown in the control signal frame configuration diagram of FIG. 1, using predetermined slots in a frame (#0 and #8 in FIG. 1), by the timing t_(offset) which is offset from the top of a slot by a predetermined time, a base station apparatus transmits the first synchronization channel (Primary Synchronization Code Channel: Cp) that is common to all cells, and the second synchronization channels (Secondary Synchronization Code Channel: Cs) that carry three codes to express a code group, simultaneously. As for the second synchronization channels, selecting 3 types from 17 types gives 4913=17³ combinations, and out of these, the 32 least error detection types are used to express a code group.

[0005] Moreover, a second synchronization channel, Csj (j=1, 2, 3), is subjected to modulation that is 90°×n (n=0, 1, 2, 3) to the phase of the first synchronization channel Cp upon transmission. b_(j) in FIG. 1 denotes the phase rotation amount in each second synchronization channel in relative to the phase of the first synchronization channel Cp.

[0006] As for the cells, these can be selected from Case 1, where synchronization channels are transmitted using one portion (the kth slot) of a frame (10 ms), and from Case 2, where synchronization channels are transmitted using two portions (the kth slot and the k+8th slot) of a frame (k is a whole number from 0 to 7).

[0007] When performing an initial cell search, for the first step, a communication terminal apparatus performs the correlation calculation of the first synchronization channel, and detects the timing giving the largest correlation value (hereinafter “peak timing”) as a slot timing.

[0008] Next, for the second step, the communication terminal apparatus performs the correlation calculation of 17 types of second synchronization channels and identifies the three types of second synchronization channels being transmitted from a base station apparatus. When identifying these second synchronization channels, the communication terminal apparatususes 4-frame signals for Case 1, and 2-frame signal for Case 2. Then, the communication terminal apparatus identifies the code group assigned to its cell based on the phase rotation amount in four frames of the identified three types of second synchronization channels and the time offset value t_(offset) of the synchronization channels from the top of a slot, and thus detects the timing of a frame top.

[0009] Finally, for the third step, the communication terminal apparatus performs the correlation calculation of the four types of mid-ambles belonging to the identified code group, and identifies the mid-amble and scrambling code assigned to its cell. Incidentally, to improve the characteristics of this mid-amble detection, the communication terminal apparatus performs wireless communication while synchronizing with the base station through AFC (Automatic Frequency Control).

[0010] In a mobile communication system of a W-CDMA/TDD scheme, thus, a communication terminal apparatus performs an initial cell search (scrambling code identification) in three steps.

[0011] Now, in the above cell search, a communication terminal apparatus is not frequency-synchronized with a base station, and so the oscillators in these apparatus have different oscillation frequencies, thus measuring one frame time differently.

[0012] However, since a conventional cell search method does not take the difference between frame times measured by a communication terminal and a base station apparatus into consideration, in the second step, correlation calculation is performed at a wrong timing that is off the peak timing.

[0013] For instance, if the one-frame time measured by a base station serves as a standard, and, in comparison thereto, the one-frame time measured by a communication terminal is shorter than that by the above base station by α[s], the communication terminal apparatus, in the second step, develops the difference from the peak timing by α[s] for every frame. If second synchronization channels are identified over four frames, a difference of maximum 4α[s] can result. In case this error develops large (greater than ½ chip time, for instance), the communication terminal apparatus becomes incapable of peak detection and detecting second synchronization channels.

[0014] Moreover, if the difference between the oscillation frequencies of the oscillators in a communication terminal apparatus and a base station apparatus grows large, the accuracy of synchronization detection in the early stages of AFC in the third step decreases, thereby making accurate mid-amble detection difficult.

DISCLOSURE OF INVENTION

[0015] One of the primary objects of the present invention is to provide apparatus and methods for cell search for use in mobile communication systems of W-CDMA/TDD schemes that enable accurate identification of second synchronization channels and mid-ambles even where a base station gives a different measurement of a frame time.

[0016] The above object can be achieved by, in the second step of a W-CDMA/TDD scheme-based cell search, comparing the correlation calculation results of several timings and selecting the second synchronization channels from the timings that correspond to the largest correlation calculation results.

BRIEF DESCRIPTION OF DRAWINGS

[0017]FIG. 1 is a frame configuration diagram of control signal at base station apparatus;

[0018]FIG. 2 is a block diagram showing a configuration of a cell search apparatus according to the embodiment of the present invention; and

[0019]FIG. 3 shows in detail the second step in a cell search apparatus according to the embodiment of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0020] An embodiment of the present invention will be described below with reference to the accompanying drawings.

[0021]FIG. 2 is a block diagram showing a configuration of a cell search apparatus according to the embodiment of the invention. The following explanation concerns Case 1, where a frame (10 ms) transmits synchronization channels using one portion (kth frame) thereof.

[0022] Antenna 101 receives a career frequency signal transmitted from a base station. RF receiver 102 subjects the signal received by antenna 101 to various processings including down-conversion, amplification, and A/D conversion, so as to obtain a baseband digital signal.

[0023] PSCH correlator 103 calculates the correlation between the output signal from RF receiver 102 and the first synchronization channel. Peak detector 104 detects the timing where the correlation with the first synchronization channel becomes maximum (hereinafter “first peak timing”) and outputs the detection result to timing controller 105 and to frequency corrector 110.

[0024] The above processings in PSCH correlator 103 through peak detector 104 are the first step of a cell search.

[0025] Based on the first peak timing detected in peak detector 104 and based on the second peak timing detected in SSCH identifier 108, which will be described later, timing controller 105 controls the timing to take correlation values out of memories 107˜107-n. Each of SSCH correlators 106-1˜106-n is assigned one of the second synchronization channels, and calculate the correlation between the assigned second synchronization channel and the output signal from RF receiver 102 on a per (¼) Ts basis. “Ts” denotes the time of one chip. Memories 107-1˜107-n store four frames of the correlation values output from corresponding SSCH correlators 106-1˜106-n.

[0026] SSCH identifier 108 takes the correlation values out of memories 107-1˜107-n by the timing instructed by timing controller 105, identifies three second synchronization channels of the same timing, that correspond to the three largest correlation values, and outputs these to code group identifier 109. Moreover, as a second peak timing, SSCH identifier 108 outputs the timing where the sum value of the identified second synchronization channels become the largest to timing controller 105 and to frequency corrector 110. Code group identifier 109 identifies the code group assigned to its cell based on the phase rotation amount in four frames of the three second synchronization channels identified in SSCH identifier 108 relative to the first synchronization channel, and thus detects the timing of a frame top.

[0027] The above processings in timing controller 105 through group identifier 109 are the second step of a cell search. The above second step in the cell search apparatus according to the present embodiment will be described in more details later.

[0028] Based on the first peak timing and the second peak timing, frequency corrector 110 calculates the difference relative to the frame time measured in the base station, and converts this calculated difference into a frequency difference. Then, frequency corrector 110 corrects the frequency of the oscillator in RF receiver 102 in such a way that corrects the frequency difference. Incidentally, detailed examples of calculation in frequency corrector 110 will be described later.

[0029] Mid-amble correlators 111-1˜111-m calculate the correlation between the mid-amble belonging to the identified code group and the output signal from RF receiver 102, and outputs the correlation value to mid-amble identifier 112.

[0030] Mid-amble identifier 112 identifies the mid-amble from those corresponding to the largest correlation values output from mid-amble correlators 111-1˜111-n. Moreover, the mid-ambles and scramble codes are in one-to-one pairs, and in accordance therewith identifier 112 identifies the cell's scrambling code.

[0031] The processings in frequency corrector 110 through mid-amble identifier 112 are the third step of a cell search. The cell search apparatus of FIG. 2 completes an initial cell search by implementing the above first through three steps (i.e. scrambling code identification).

[0032] Next, the second step in the cell search apparatus of the present embodiment will be described in detail with reference to FIG. 3. In FIG. 3, frame F₀ denotes the frame where the first peak timing T_(f0) is detected, and frame F_(i) is the ith frame from frame F₀.

[0033] The window width in timing controller 105 is set in advance to a certain width (Ts/2 in FIG. 3). Then, timing controller 105 instructs memories 107-1˜107-n on the timing Tf₁ which is one frame time after the first peak timing Tf₀, and the timings (Tf₁−Ts/4) and (Tf₁+Ts/4) that corresponds to the front and rear ends of the window centering around the timing Tf₁.

[0034] Memories 107-1˜107-n output the correlation values at the timings Tf₁, (Tf₁−Ts/4), and (Tf₁+Ts/4) to SSCH identifier 108.

[0035] With frame F1, with reference to the correlation values output from memories 107-1˜107-n at Tf₁, SSCH identifier 108 identifies the three largest second synchronization channels. Moreover, at the timings Tf₁, (Tf₁−Ts/4), and (Tf₁+Ts/4), SSCH identifier 108 adds the correlation values of the three identified second synchronization channels, detects the timing giving the largest sum as a second peak timing, and outputs the second peak timing to timing controller 105 and to frequency corrector 110. In the case of FIG. 3, the sum is maximum at the timing of (Tf₁+Ts/4), and so SSCH identifier 108 sets a second peak timing on the timing (Tf₁+Ts/4). Incidentally, since the addition of the correlation values can reduce the impact of noise, and accordingly, detecting a maximum timing by adding correlation values and using the result thereof is more accurate than performing such detection using one correlation value.

[0036] That the sum of the correlation values is maximum at the timing (Tf₁+Ts/4) denotes that the frame time measured in the cell search apparatus is shorter than that by the base station.

[0037] For instance, if a frame time measured in a cell search apparatus is shorter than that by a base station by α[s], in case of a typical cell search apparatus, the timing difference in correlation calculation becomes 2α[s] in frame F₂. In contrast, if a cell search apparatus according to the present invention is used, the timing difference upon correlation calculation in frame F₂ becomes (2α−Ts/4) [s], and the difference is thus reduced compared to the above typical cell search apparatus.

[0038] Timing controller 105 instructs memories 107-1˜107-n on the timing T_(f2), and the timings (T_(f2)−Ts/4) and (Tf₂+Ts/4) corresponding to the front and rear ends of the window centering around the timing T_(f2).

[0039] Memories 107-1˜107-n output the correlation values at T_(f2), (T_(f2)−Ts/4), and (T_(f2)+Ts/4) to SSCH identifier 108.

[0040] With frame F2, with reference to the correlation values output from memories 107-1˜107-n at the timing of Tf₂, SSCH identifier 108 identifies the three largest second synchronization channels. Moreover, at the timings Tf₂, (Tf₂−Ts/4), and (Tf₂+Ts/4), SSCH identifier 108 adds the correlation values of the three identified second synchronization channels, and outputs the timing giving the largest sum as another second timing to timing controller 105 and frequency corrector 110. In the case of FIG. 3, the sum is maximum at the timing of Tf₂, and so SSCH identifier 108 sets another second peak timing on the timing Tf₂. Thereafter, with frames F₃ and F₄, the cell search apparatus identifies the second synchronization channels in the same manner as in the processings for frame F₂.

[0041] Comparison of the correlation values at several timings including the standard timing and the setting of a new standard timing upon the timing that gives the largest correlation value thus makes it possible to save timing differences upon correlation calculation within a given range, so that peak detection and second synchronization channel identification in the second step can be performed with certainty.

[0042] Incidentally, in the above explanation, the window width is set centering around a standard timing that is one frame time behind the first or the second peak timing, and, with respect to the three timings including the standard timing and the timings corresponding to the front and rear ends of the window, the sums of the correlation values are compared. However, the present invention is by no means limited to the above and the comparison of the sums of correlation values can be performed with respect to a number of timings within a predetermined time range centering around a standard timing.

[0043] Next, the method of calculation in frequency corrector 110 will be described in detail assuming the case of FIG. 3.

[0044] In FIG. 3, the second peak timings in the frames are respectively (Tf₁+Ts/4), Tf₂, (Tf₃+Ts/4), and Tf₄, indicating that error of Ts/2 has occurred over four frames. Where one frame is 10 ms and Ts is (1/3.84)×10⁻⁶ ms, this gives error of (1/(4×2×3.84))×10⁻⁶ ms per frame. If this is converted into frequency difference, using ppm (10⁻⁶) for the frequency difference unit, the result will be (1/(4×2×3.84)) ppm. Such calculation is performed in frequency corrector 110, and the frequency of the oscillator in RF receiver 102 is corrected in such a way that corrects the frequency difference. Incidentally, given that voltage control oscillators are in common use, frequency corrector 110 supplies voltage that is equivalent of the frequency difference to the oscillator.

[0045] By correcting the frequency difference obtained thus and setting the initial level of AFC, the accuracy of detection in the early stages of AFC can be improved, so as to enable accurate mid-amble detection.

[0046] Furthermore, referring to the third step, receiving second synchronization channels that are not used for AFC alone increases power consumption. In contrast, the present embodiment solves the above problem as second synchronization channels become unnecessary during AFC in the third step.

[0047] As obvious from the above description, according to the present invention, the differences between the timings of correlation calculation can be constantly saved within a given range, so that second synchronization channels and mid-ambles can be accurately identified.

[0048] The present application is based on Japanese Patent Application No. 2001-239177 filed on Aug. 7, 2001, the entire content of which is expressly incorporated herein by reference.

INDUSTRIAL APPLICABILITY

[0049] The present invention is suitable for use with wireless communication terminal apparatus in mobile communication systems of W-CDMA/TDD schemes. 

1. A cell search apparatus, comprising: a first correlator that performs a correlation calculation of a first synchronization channel; a peak detector that detects a first peak timing based on a result of the correlation calculation in said first correlator; a second correlator that performs the correlation calculation of a second synchronization channel on a predetermined time interval; a second synchronization channel identifier that compares results of the correlation calculation in said second correlator at a plurality of timings and identifies the second synchronization channel from the timings that give largest correlation calculation results; and a timing controller that gives instruction to said second synchronization channel identifier on the plurality of timings to perform the correlation calculation.
 2. The cell search apparatus according to claim 1, wherein the second channel identifier detects a second peak timing, said second peak timing being the timing that gives the largest correlation calculation result among the plurality of timings; and wherein, based on the first peak timing and the second peak timing, the timing controller sets the plurality of timings that said second synchronization channel identifier is instructed on.
 3. The cell search apparatus according to claim 1, wherein the second timing controller instructs the second channel identifier on the plurality of timings from within a predetermined time range centering around a standard timing that is one frame time after the first or the second peak timing.
 4. The cell search apparatus according to claim 1, wherein the second synchronization channel identifier detects a second peak timing through comparison of sums of correlation values of a plurality of the second synchronization channels identified at the timings instructed by the timing controller.
 5. The cell search apparatus according to claim 1, further comprising a frequency corrector that, with reference to the first and second peak timings, calculates a difference of a frame time measured by said cell search apparatus relative to the frame time measured by a base station apparatus of a communication partner, converts said difference to a frequency difference, and changes an oscillator frequency in such a way that corrects said frequency difference.
 6. A wireless communication terminal apparatus comprising a cell search apparatus, said cell search apparatus comprising: a first correlator that performs a correlation calculation of a first synchronization channel; a peak detector that detects a first peak timing based on a result of the correlation calculation in said first correlator; a second correlator that performs the correlation calculation of a second synchronization channel on a predetermined time interval; a second synchronization channel identifier that compares results of the correlation calculation in said second correlator at a plurality of timings and identifies the second synchronization channel from the timings that give largest correlation calculation results; and a timing controller that gives instruction to said second synchronization channel identifier on the plurality of timings to perform the correlation calculation.
 7. A cell search method comprising: a first step, wherein a correlation calculation of a first synchronization channel is performed so as to detect a first peak timing that gives a largest correlation value as a slot timing; a second step, wherein correlation calculations of all second synchronization channels are performed and a code group assigned to a cell where an apparatus implementing the method lies is identified, so as to detect a timing of a frame top; and a third step, wherein the correlation calculation of a mid-amble belonging to the identified code group is performed, so as to identify the mid-amble and a scrambling code assigned to the cell where the apparatus lies, wherein, in the second step, results of the correlation calculations at a plurality of timings are compared so as to identify the second synchronization channels from those corresponding to the timings that give largest correlation calculation results.
 8. A cell search method, wherein the third step is performed such that a difference of a frame time relative to the frame time measured in a base station apparatus of a communication partner is calculated and converted to a frequency difference, and an oscillator frequency in such a way that said frequency difference is corrected. 